Refuse a black mark (7)
I believe the answer is:
abstain
'refuse' is the definition.
(I've seen this before)
'a black mark' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'mark' becomes 'stain' (synonyms).
'a'+'b'+'stain'='ABSTAIN'
(Other definitions for abstain that I've seen before include "Choose not to do something" , "Take no part" , "Hold off" , "Go without" , "decline at voting time" .)